Recently, as the demand for high-quality large-screen TVs such as high definition TVs has increased, displays suitable for such TVs, such as Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), and Plasma Display Panel (PDP), have been developed.
CRTs have been widely used as TV displays and excel in terms of resolution and picture quality. However, the depth and weight increase as the screen size increases. Therefore, CRTs are not suitable for large screen sizes exceeding 40 inch. LCDs consume a small amount of electricity and operate on a low voltage. However, producing a large LCD screen is technically difficult, and the viewing angles of LCDs are limited.
On the other hand, it is possible to make a PDP with a large screen with a short depth, and 50-inch PDP products have already been developed.
PDPs are divided into two types: Direct Current (DC) and Alternating Current (AC). Currently, PDPs are mainly AC-type since they are suitable for large screens.
AC surface-discharge type PDP, a typical AC-type PDP, is typically composed of a front panel and a back panel to each of which electrodes are attached so that the electrodes of both panels face each other. A space between the front panel and the back panel is divided into a plurality of spaces by barrier ribs. The plurality of spaces between these barrier ribs are each filled with discharge gas and any of red, green, and blue all fluorescent substances. When a driving circuit applies a voltage to each electrode to cause a discharge, ultraviolet light is emitted. The ultraviolet light excites fluorescent substances. The excited fluorescent substances emit red, green, and blue lights. The emitted light of these colors forms images on the screen.
Typically, such PDPs are manufactured by the following procedure. The barrier ribs are disposed on a surface of the back panel; fluorescent substance layers are formed in grooves between the barrier ribs; the front panel is laid on top of the barrier ribs to form a surrounding unit (the front panel and the back panel bonded together with inner space in between); the rim of the surrounding unit, that is, the front and back panels, is sealed with a sealing material; gas is exhausted from the inner space to produce a vacuum; and the inner space is filled with a discharge gas.
The sealing material is typically a low-melting glass that softens by heat. A mixture of the low-melting glass and a binder is applied by a dispenser or the like to the rim of either the front panel or the back panel before the surrounding unit is constructed by putting the panels together. In the sealing process, the panels are bonded by heating the panels to a temperature higher than the softening point of the low-melting glass while the rim of the surrounding unit covering the applied sealing material and the outermost area is fixed by clips or the like.
However, the PDPs manufactured with such a method have gaps between the barrier ribs and the front panel. The gaps vary from barrier rib to barrier rib or from point to point on each barrier rib. The reason for this is considered as follows. (1) Variations of the barrier ribs in height are generated in the barrier rib formation process in which the material for the barrier ribs are placed on the back panel. (2) The panels and the barrier ribs are distorted in heating processes such as the processes for baking the barrier ribs, fluorescent substance, electrodes, and dielectric layer, and the sealing glass layer temporary baking process which are performed before the sealing process.
Further, in the sealing process of PDP, the rim of the front panel and the back panel is fixed by fastening tools such as clips so as to prevent displacement of the panels after they were positioned to face each other. However, such fastening of the rim tends to generate a gap between the top of the barrier ribs and the front panel at the center by the action of a lever. In addition, unequal gaps are often formed since the pressures given by the fastening tools are different.
In the PDPs manufactured through the sealing process with such gaps, crosstalks often occur when the PDPs are activated, or noises often occur between the barrier ribs and the panels due to vibration of the panels caused by the u discharge or the like.
Japanese Utility Model Publication No. 1-113948 discloses a technique in which a low-melting glass is applied to the top of the barrier ribs before the front and back panels are positioned to face each other and bonded together. When the front panel is bonded with the whole top of the barrier ribs using this technique, the surrounding unit does not expand even if the inner space is filled with a discharge gas at high pressure. Also, the gaps between the barrier ribs and the front panel are filled with the sealing material. Accordingly, the technique solves the problem of the vibration.
However, in reality, it is difficult to bond the whole top of the barrier ribs with the front panel. A part of the top of the barrier ribs often remains unattached. Accordingly, this technique is not sufficient to solve the problem of the pressure. Especially, when there are variations of the barrier ribs in height on the back panel, many parts remain unattached. When this happens, it is impossible to obtain sufficient resistance to pressure.
There is another conventional method in which a set of the front and back panels is heated for sealing while a weight such as a stone is placed at the center thereof. However, according to this method, more energy is required for the heating since the weight on the panels is also heated. The heating temperatures for surrounding units tends to be unequal. It is difficult to use this technique for the production of large-screen PDPs.
There is another requirement for the production of the panels. Typically, a vacuum pump or a discharge gas-cylinder is connected to an exhaust pipe attached to the surrounding unit. The exhaust pipe is chipped off later by a burner or a heater. A reliable method of chipping off the exhaust pipe is desired.